1. Field of the Invention
The subject invention generally pertains to folding doors and more specifically to powered folding doors.
2. Description of Related Art
Doors that are powered open and closed as opposed to being manually operated are used in a variety of applications. Some doorways are so large that opening and closing a large, heavy door manually would be physically difficult. At doorways frequently traveled by vehicles, a driver can easily operate a powered door without having to leave the vehicle. And a door separating two areas where in one the environment (e.g., temperature, humidity or cleanliness) is controlled and the other is not, it can be important to minimize the time that the door is open to avoid degrading or contaminating the controlled environment. For example, a warehouse having refrigerated/freezer areas for storing large containers or pallets of frozen foods, produce or other perishable foods is an ideal application for a rapidly moving powered door, as the areas' temperature is controlled and the goods are frequently delivered and removed through the doorway by a forklift truck.
Many doors, such as horizontal sliding, vertically operated and conventional swinging doors require a significant amount of dead-space into which the door extends when open. The wasted space may need to be off to one or both sides of the doorway, overhead, or immediately in front of the door. However, accordion-style folding doors fold upon themselves as they open, which make them suitable for a wide variety of installations.
Folding doors typically include several side-by-side vertically elongated panels that hang from an overhead track. Often the panel's vertical edges are hinged to each other in an accordion fashion. An overhead chain and sprocket drive unit opens or closes the door by pulling at least one of the panels along the track while the others follow. With enough horsepower door operation can be quite rapid, but its speed may be limited by insufficient rigidity at the interface coupling the drive unit to the door panels.
In some cases the rigidity is improved by replacing part of the drive unit's chain with two rigid elongated drawbars that do not stretch, sag or whip around as freely as a chain. One example of such a folding door is disclosed in U.S. Pat. No. 5,295,527, which is specifically incorporated by reference herein. The patent discloses a folding door that has two narrow drawbars running generally parallel to the overhead track and each being rigidly coupled to a trolley from which one of the door panels hang. A chain (powered by a reversible motor-driven sprocket) moves each drawbar longitudinally to move their respective trolley, and the door panel hanging from it, back and forth along the track to open and close the door. Further rigidity is achieved by having the drawbars slide within close-fitting horizontal guide sleeves that are fixed in relation to the track. Although effective, the factor limiting the speed of the door can then become the drawbar's ability to resist the torque or bending moments created by changes in the driven panel's inertia as the door quickly accelerates or decelerates upon approaching and departing from its open and closed positions.
In closing the door, for example, as the driven trolley quickly stops at the closed position, the momentum of the door panel hanging from that trolley tends to keep the panel moving. Since the panel's trolley has stopped, the panel tends to swing about the trolley in the direction it was traveling. The swinging action applies a torque to the trolley, which is transmitted to the drawbar coupled to it. Since the drawbar is kept from rotating within a fixed horizontal guide sleeve, the drawbar is subjected to a substantial bending stress generally between where the drawbar enters the sleeve and where the drawbar connects to the trolley. A similar adverse effect occurs as the door decelerates/stops at its open position and also occurs as the door quickly accelerates from its closed or open positions. When the bending stress is sufficient to permanently deform the drawbar, the bent drawbar tends to repeatedly bind within the close sliding-fit of the guide sleeve. This places an added load on the drive unit, which can lead to premature wear and failure of the drive unit and other parts of the door. Although the drawbar could be made to withstand higher bending stresses by increasing its diameter, a larger diameter drawbar adds weight, size, product cost, frictional drag in the guide sleeve and inertia of its own that may further limit the speed of the door.